Can a Space Marine army consist of units from different chapters? has a generic question to which the answer varies by edition (quite significantly in this case).
How should we handle tagging of board games with different editions? says we should use edition tags:
A new tag should be added for the new edition and existing tags can be updated as needed if they only apply to an older edition. There will always be questions that can apply to all editions and those should still get a general tag.
Should we tag questions like the linked WH40k question with an old edition, and have new questions for new editions, or should we have a one-stop shop type answer for that question?